Pages

Thursday, July 25, 2013

NASA’s planned Mars 2020 rover likely will both continue the
astrobiological exploration of Mars begun by the Curiosity rover and provide
stepping stones to the next stages of Martian exploration.

Two weeks ago, NASA’s Mars 2020 rover Science Definition Team (SDT)
delivered its report recommending the science goals for the mission. Probably to the surprise of no one, the team
recommended essentially the same science goals as had several previous SDT’s on
what NASA’s next mission to Mars should do.
Like the Curiosity rover currently on Mars and the planned European and
Russian ExoMars rover mission, the 2020 rover will look for clues as to whether
Mars ever contained the conditions to enable life and whether traces of life or
pre-biotic chemistry remain.

The mission will also provide an important transition to the next
phases of Mars exploration by caching samples that could eventually be returned
to Earth and testing technologies for future human and robotic missions.

The challenge the SDT faced was how to do all of this on a budget (~$1.5B)
that with inflation may be just somewhat more than the half cost of the initial
Curiosity rover. To fit within the
budget, a key tradeoff would have to be made that will make the 2020 rover less
capable in a key respect than the Curiosity and ExoMars rovers.

The 2020 rover will be enabled by the substantial investment NASA made
in the design of the Curiosity rover and its entry, descent, and landing
system. NASA and the Jet Propulsion
Laboratory that built the Curiosity rover also retain substantial stockpiles of
spare parts and engineering expertise that can be used in rebuilding
substantial portions of the spacecraft.

I’ve read many press summaries of the SDT’s recommendations, which tend
to focus on the proposed caching of samples for possible return to Earth. If NASA follows through, this will be the
first concrete step towards a goal that Mars scientists have made their top
priority for decades. However, the
caching is just one aspect of the proposed mission.

To understand the full promise of this mission, I’ll go through each of
the SDT’s proposed goals for the mission (which closely parallel those NASA
asked the SDT to consider). First,
though, the proposed mission implementation to meet those goals makes more
sense with some background on how rovers are used as scientific platforms.

In the course of driving several kilometers, a rover will pass by
thousands of potential spots for more detailed examination. Each of those examinations, though, can take
days to weeks complete. There is a
tradeoff between driving distance, and hence number of locales that can be explored
and the number of spots where in-depth data can be gathered.

To make the best trade possible between these two goals, the mission
team employs a hierarchy of scientific assessments with those at the top taking
the least time and those at the bottom the most. First, the team uses orbital observations to
select the locales it would like to visit.
Then as the rover arrives at each locale (and also during the drives
between them), the rover’s remote sensing instruments are used to get the “big
picture.” From these images, the science
selects a small number of targets for the next level of investigation by the
contact instruments. As the name
implies, these instruments are placed in contact with a rock or soil sample
target. Instruments in this class
include the microscopic imagers and alpha-particle
X-ray spectrometers carried by the MER Spirit and Opportunity rovers and
the Curiosity rover. For a still smaller
number of extremely interesting targets, the time is taken to collect a
sample. On the Curiosity rover and the
planned 2018 ExoMars rover, these samples are delivered to highly sophisticated
analytical instruments inside the rovers for more detailed measurements. The 2020 rover will collect the samples and
place them in a cache, which may eventually be returned to Earth for more
detailed measurements than can be made within a rover. (The MER rovers do not have the capability to
collect samples.)

Rovers on Mars follow a hierarchical
strategy for selecting a small number of targets for in-depth exploration.

The SDT described their proposal for the 2020 mission in terms of
fulfilling four goals, and I’ll present their recommendations for each of those
four goals.

Goal A: “Explore an
astrobiologically relevant ancient environment on Mars to decipher its
geological processes and history, including the assessment of past
habitability.”

The 2020 rover would follow a
hierarchical strategy to explore a location on Mars believed to have been habitable. The definitive identification of past signs
of life is likely to require the testing of returned samples in Earth
laboratories.

Every NASA landed Mars mission – except the 1996 Pathfinder mission that
focused on technology demonstration – has had the goal of exploring Mars’ past
and current potential for life or pre-biotic chemistry. For the MER rovers, the goal was simply to
determine whether water – an essential ingredient for life – was present at the
surface early in Mars’ history. The
Curiosity rover is exploring Gale Crater to examine soils from many eras of
Martian history to determine whether or not environments for life existed and
to determine whether biosignatures of past life remain. The 2018 ExoMars rover will explore another
site on Mars for its astrobiology potential.

The SDT has proposed that the 2020 rover continue the strategy and
pursue astrobiology as the mission’s defining goal. Its proposed strategy breaks into two
parts. The first is to have the rover
carry a suite of instruments capable of a exploring site’s geologic history
in-depth with an emphasis on how that history affected the possible presence of
past life.

The team proposes that the rover carry multispectral cameras for
obtaining images and an imaging spectrometer for analyzing composition across
entire sites. These instruments would
provide the context to interpreting each locale’s history as well as allowing
the science team to select specific targets for more detailed exploration.

For most of the contact science, the SDT is proposing that the rover
carry a new generation of instruments.
Current Mars rover contact spectrometers measure mean composition over an
approximately two centimeter contact area.
The new generation of instruments under development can make composition
measurements for spots as small as a tenth of a
millimeter. With that resolution,
the contact spectrometers would make dozens to hundreds of measurements across
the contact area.

If you take a close look at soils and most rocks, you’ll see that most
are composites of many fragments that each have their own geological story to
tell. The contact spectrometers that are
likely to be proposed for the 2020 rover will be capable of exploring each of
those fragments individually. (The SDT
also proposes that the rover carry an imaging microscope to study the morphology
and texture of each contact area.)

The remote sensing and contact instruments listed above are included in
the baseline recommendations and are expected to be affordable at the low end
of the expected budget (~$90M to $125M) for the science instruments. If the budget becomes plusher, the SDT
recommends two additional instruments to study the shallow subsurface beneath
the rover. A ground penetrating radar
would detect subsurface rock and soil layers, providing better context for
understanding the geology exposed at the surface. A gamma ray spectrometer would measure the
composition of soil in the upper few centimeters and could alert scientists to
interesting substances just below the upper veneer of soil.

A capable instrument suite enables scientific exploration; the rover
still must be delivered to a location that orbital instruments show might have
been a location for life or pre-biotic chemistry. A number of such locations are known, and
more are being searched for. However,
these sites often lie within rough terrains with just a small area free of
large rocks that would end the mission should the rover be unlucky enough to
land on one. The 2020 rover mission will
inherit the precision landing system developed for the Curiosity rover that
reduced the area of the landing ellipse to a fraction of what it had been for
previous landers.

The SDT recommends shrinking that ellipse further to allow more landing
sites to be considered. On past missions,
the parachute has opened at the earliest possible time during the descent. For the 2020 descent, the SDT recommends that
the entry system have the ability to vary the time of opening based on its
estimate of its position relative to the landing zone. This relatively simple
enhancement could reduce the size of the landing ellipse by 25% to 50%.

Many potentially interesting astrobiology sites on Mars lack any area
the size of a landing ellipse free of large rocks or dangerously steep
slopes. A second enhancement the SDT asked
NASA to consider is terrain recognition navigation (TRN) that would enable the
lander to compare images of the landing area stored on board with real-time
images taken during the descent. This
capability would allow the descent system to determine its actual location and
steer free of hazardous terrain in the moments of final descent.

Examples of potential biosignatures and
measurements the 2020 rover could make to find them.

Much of the scientific attraction of Mars comes from its preservation
of ancient surfaces and rocks that might retain records of conditions that
could have led to life or even records past life itself. The second proposed goal for the 2020 mission
would have the rover actively assess whether biosignatures could have been
preserved and to search for those biosignatures.

What would be a biosignature? If
we were extremely lucky, it might be fossilized layers from algae-like
micro-organisms visible to the cameras.
More likely, it would be the alteration of rock or soil chemistry in a
way that would be best explained by complex organic chemistry or the actions of
life. Again, if we were lucky, it could
be the presence of organic matter preserved for billions of years.

The rover would seek biosignatures using all of the instruments listed
above and with one or two instruments that would be selected for their ability
to detect organic material. The Viking
and Phoenix landers and the Curiosity rover (and the future ExoMars rover) have
relied on sophisticated analytical instruments such as mass spectrometers to
detect organic molecules. These instruments
are capable of much more sophisticated measurements than is possible with
contact instruments that must operate directly in the harsh Martian
environment. Analytical instruments have
soil samples delivered to them where they can be analyzed with numerous
techniques and altered through heating or wetting to release gasses or induce
chemical reactions.

Unfortunately, the budget for the 2020 rover does not include funding
for analytical instruments. Instead, the
SDT proposes that the rover carry one or two spectrometers capable of detecting
the presence organic matter. The authors
of the SDT report are clear that the strategy they propose will result in the
2020 rover having significantly less capabilities to analyze potential organic
material than other missions with analytical instruments.

The SDT points out a compensating new capability for the 2020 rover:
sample caching. As described below, if
the cache is eventually returned to Earth, terrestrial laboratories could
perform far more sophisticated measurements than would ever be possible on a
rover or lander.

Priorities for samples to be
collected. The E2E-iSAG was a previous
mission assessment for a rover mission focused on selecting and caching
samples.

Returning a carefully selected set of Martian samples has long been a
goal of the Mars science community. The
last planetary Decadal Survey ranked a mission to select and cache a set of
samples for future return to Earth as its highest priority large mission for
the coming decade. A follow on mission
would collect the samples and take them into Martian orbit, and a third mission
would retrieve the samples from that orbit and bring them back to Earth.

The President’s Office of Management and Budget balked at beginning a sequence
of missions that in combination could cost $6B to $8B. They agreed to the 2020 rover mission to
continue the in situ exploration of
Mars and to demonstrate technical progress towards future robotic and manned
missions, including caching samples.

The SDT concluded that the difference between demonstrating the
technical capability to select and cache samples and actually leaving a
returnable cache would be minimal. They
recommend the rover assemble a sample cache of up to 31 to 38 five centimeter long core samples of rock and soil acquired by the
rovers drill. Once collected, the cache
would be placed on the surface for a future mission to collect in a few years
or even a few decades.

When the 2020 rover mission was approved by the President’s office, one
of the requirements was that it demonstrates technologies that would be useful
for future robotic and human missions. The SDT recommended four options be considered
(in priority order):

Demonstrate the ability to capture and compress
Martian air (which is primarily CO2) and extract liquefied oxygen
for use as the oxidizer for the fuel for future a robotic or manned ascent stage
from the Martian surface (commonly called in-situ resource utilization or ISRL).
While many parts of this technology can
be demonstrated on Earth, key issues of collecting CO2 under varying
dust conditions, winds, atmospheric pressure, and temperatures can be best
demonstrated on Mars

Better instrument the entry and descent system
to collect information on the conditions of descent and parachute
performance. (The Curiosity entry and
descent system collected extensive information during its landing; the proposed
2020 system would collect that information and new information.) The technologies discussed earlier to reduce the
risk of landing by timing the parachute opening and using terrain recognition
would also benefit future missions.

A biomarker system to “demonstrate detection of
microbial contamination for future human missions.”

Concluding Thoughts

Capabilities of the 2020 rover proposed
by the SDT. Boxes without ‘+’ represent
the recommended threshold capabilities below which the mission might not
deliver sufficient value for the investment.
Boxes with ‘+’ represent highest priority additions beyond the threshold
capabilities. If more funding than the
SDT were available, the mission could be further enhanced with either more
capable instruments or additional instruments such as a weather station or additional
instruments to characterize any organic matter.

The mission proposed by the SDT would meet the goals set out for a
caching rover as the top priority in the last Decadal Survey. It would also meet the goals recently set out
by NASA for demonstrating key technologies for future missions.

At the same time, even if the samples not collected, the rover would
carry out intensive geological and astrobiological exploration at a fifth site
on Mars. (The Viking landers explored
two sites in the 1970s, the Phoenix lander explored the ice-rich northern
plains, the Curiosity rover is exploring Gale Crater now, and the ExoMars rover
will presumably explore yet another site.)
The new generation of contact instruments that will be ready for the
2020 rover will allow exploration of the composition of Martian soils and rocks
at micro-scales that previous missions have not be able to do. This is an exciting new capability.

While the SDT report doesn’t spend much time on the possible weather
station that the 2020 rover may carry, I think this would be an important
addition. Scientists have long wanted to
get a network of metrological stations on Mars to better explore weather
patterns. In 2020, there may still be
three functioning weather stations already on Mars: Curiosity, NASA’s InSight
lander, and the Russian surface station planned for the ExoMars mission. A fourth station would be an important
addition.

The 2020 rover is a mission that will be done on a tight budget. The highly capable analytical laboratories of
the Viking and Phoenix landers and the Curiosity and ExoMars rovers would not
fly on the 2020 rover. The capabilities
the SDT recommends for the 2020 rover meet all the requirements previous SDT’s
have laid out for a rover mission focused on sample selection and caching. However, the 2020 rover would have less
capability for science on Mars for characterizing organic matter and other
volatiles than Curiosity or the ExoMars rover.

Of course, the need for the analytical laboratory would go away if the
sample cache is returned to Earth laboratories.
The 2020 rover would make the investment in the first crucial step,
selection and caching of samples, of the Holy Grail of Mars science: returning
samples. How long might those samples
sit on the surface of Mars before being collected? I expect that that will be a question for how
compelling the discoveries by the rover’s instruments are and the generosity of
future governments. And it may not be
American craft or only American craft that collect and return the samples. In the coming two decades, several space
agencies are likely to have the technology to participate in the sample return.

A second bet being made is to leave out a deep drill such as the
ExoMars rover will carry. The drill
proposed for the 2020 rover will collect samples five
centimeters (about two inches) in length, similar to that carried by the
Curiosity rover. This may not be deep
enough to get below the surface radiation that
is believed to destroy organic matter at Mars (see this post). The ExoMars drill was
designed with this problem in mind and will reach up to two meters below the
surface. Adding a similar drill to the
2020 rover would require a substantial modification to the Curiosity rover design
isn’t possible within the budget.
However, if the Curiosity rover doesn’t find organic matter and the
ExoMars rover does but deeper beneath the surface, then the bet on the shorter
drill will look problematic.

Even with the current budget realities, though, the SDT has proposed a
highly capable, exciting mission. NASA’s
officials warmly received the recommendations, indicating that their final
choices for the mission are likely to be similar to those recommended by the
SDT, but some changes are possible.

A key decision by NASA will be on whether to fund the sample collection
and caching system and the sophistication of that system. Current news reports indicate that it intends
to fly this system.

The next step for the mission will be for NASA to issue a call to
solicit instrument proposals this coming fall.
The type of instruments NASA says it is interested in receiving
proposals for is likely to be the definitive statement on the mission’s
scientific goals.

For more information on the SDT’s recommendations you can read these
documents.

Sunday, July 14, 2013

Long time readers of this blog know that in the past, I frequently
wrote to give updates on the progress of NASA’s budget as it moved through
Congress to final approval. Before this
year, no one else was providing this kind of coverage focused on the planetary
program. Now Casey
Drier at the Planetary Society regularly posts updates, and I recommend
that you add the Society’s blogs to your regular reading. This will free up more time for me to write
longer stories about potential and planned missions such as the one
I did recently on Uranus. (When I
started this blog, I was getting my PhD, and had one dissertation-quality
research project. Now I’m doing three to
four.) I will still write in-depth
analyses of NASA’s budget (and other space agencies when I can find detailed
information), but will wait for major milestones.

This
year, as you’ll recall, the President proposed to continue a much reduced
(~$1.2B) budget for planetary exploration in Fiscal Year 2014 compared to
budgets of ~$1.5B just a few years before.
The good news is that budget committees in both the House of
Representatives and the Senate appear to want to substantially raise the FY14
budget compared to the proposal. The
House has recommended raising the budget back to approximately $1.5B. It does so, in large part by substantially
cutting the budget for NASA’s Earth Science program. The cuts to the Earth Science program would
be, in my opinion, as devastating for that program as the cuts to the planetary
program have been at a time when human activities are dramatically changing our
planet. I believe those cuts would be
ill-advised. (Full disclosure: Part of
my research funding has come from NASA’s Earth Science program and I use data
from that program in my research.)

The proposed House budget would also reduce NASA’s overall budget to
$16.6B, a figure that was last seen in 1986.
(Casey has a
nice graphic in his post.) If you go
to one of the inflation calculators available on the web, you’ll find out that
there’s been 112% inflation since 1986. A
simple consumer inflation index, however, doesn’t fully capture the change in
costs for NASA. The march of technology
will have reduced the costs of many items and activities since 1986, but other
costs such as maintaining buildings and making payroll probably will have
increased close at a rate close to the consumer price index. Regardless of how this proposed budget
compares to 1986, it would be another in a series of cuts to NASA over the last
several years.

This year the two houses of Congress (each controlled by a different
political party) seem further apart than ever in their views of what the
Federal budget should be. I expect many
twists and turns, which you can follow on Casey’s blog. When major events occur, I will do my traditional
in-depth analyses.

Tuesday, July 9, 2013

NASA today announced the goals for its planned Mars 2020 rover mission. The mission will be based on the Curiosity rover and its entry, descent, and landing design.The science team laid out the following goals:

Explore an astrobiologically relevant ancient environment on Mars to decipher its geological processes and history, including the assessment of past habitability.

Assess the biosignature preservation potential within the selected geological environment and search for potential biosignatures.

Cache samples for a potential future sample return mission

Support technology demonstrations and weather measurements that will contribute towards a possible future human mission.

I'll read through the Science Definition Team's report this week and provide a more detailed analysis later this week.In the meantime, Casey Drier at the Planetary Society Live Blogged the press conference. I've also copied the press release below:News release: 2013-217 July 9, 2013

WASHINGTON -- The rover NASA will send to Mars in 2020 should look for signs of past life, collect samples for possible future return to Earth, and demonstrate technology for future human exploration of the Red Planet, according to a report provided to the agency.

The 154-page document was prepared by the Mars 2020 Science Definition Team, which NASA appointed in January to outline scientific objectives for the mission. The team, composed of 19 scientists and engineers from universities and research organizations, proposed a mission concept that could accomplish several high-priority planetary science goals and be a major step in meeting President Obama's challenge to send humans to Mars in the 2030s.

"Crafting the science and exploration goals is a crucial milestone in preparing for our next major Mars mission," said John Grunsfeld, NASA's associate administrator for science in Washington. "The objectives determined by NASA with the input from this team will become the basis later this year for soliciting proposals to provide instruments to be part of the science payload on this exciting step in Mars exploration."

NASA will conduct an open competition for the payload and science instruments. They will be placed on a rover similar to Curiosity, which landed on Mars almost a year ago. Using Curiosity's design will help minimize mission costs and risks and deliver a rover that can accomplish the mission objectives.

The 2020 mission proposed by the Science Definition Team would build upon the accomplishments of Curiosity and other Mars missions. The Spirit and Opportunity rovers, along with several orbiters, found evidence Mars has a watery history. Curiosity recently confirmed that past environmental conditions on Mars could have supported living microbes. According to the Science Definition Team, looking for signs of past life is the next logical step.

The team's report details how the rover would use its instruments for visual, mineralogical and chemical analysis down to microscopic scale to understand the environment around its landing site and identify biosignatures, or features in the rocks and soil that could have been formed biologically.

"The Mars 2020 mission concept does not presume that life ever existed on Mars," said Jack Mustard, chairman of the Science Definition Team and a professor at the Geological Sciences at Brown University in Providence, R.I. "However, given the recent Curiosity findings, past Martian life seems possible, and we should begin the difficult endeavor of seeking the signs of life. No matter what we learn, we would make significant progress in understanding the circumstances of early life existing on Earth and the possibilities of extraterrestrial life."

The measurements needed to explore a site on Mars to interpret ancient habitability and the potential for preserved biosignatures are identical to those needed to select and cache samples for future return to Earth. The Science Definition Team is proposing the rover collect and package as many as 31 samples of rock cores and soil for a later mission to bring back for more definitive analysis in laboratories on Earth. The science conducted by the rover's instruments would expand our knowledge of Mars and provide the context needed to make wise decisions about whether to return the samples to Earth.

"The Mars 2020 mission will provide a unique capability to address the major questions of habitability and life in the solar system," said Jim Green, director of NASA's Planetary Science Division in Washington. "This mission represents a major step towards creating high-value sampling and interrogation methods, as part of a broader strategy for sample returns by planetary missions."

Samples collected and analyzed by the rover will help inform future human exploration missions to Mars. The rover could make measurements and technology demonstrations to help designers of a human expedition understand any hazards posed by Martian dust and demonstrate how to collect carbon dioxide, which could be a resource for making oxygen and rocket fuel. Improved precision landing technology that enhances the scientific value of robotic missions also will be critical for eventual human exploration on the surface.

Sunday, July 7, 2013

Given my interest in
future planetary missions, I regularly look through lists of missions submitted
to space agency mission selection competitions.
I also read through the abstracts of mission concepts presented at the many
planetary science and engineering conferences each year. Uranus is trending.

Why the interest now? First, the 2011 Decadal Survey ranked a $2B
Uranus orbiter and probe mission as a priority to launch in the coming
decade. (Alas, new budget realities make
any such mission look 20 years away or more now.) Second, the Uranus-sized worlds are proving
to be common in other solar systems and may be the most common type of planet
in the galaxy. Our only up close
examinations of planets in this class were the flybys of Uranus and Neptune in
the 1980s by the Voyager 2 spacecraft that carried 1970s vintage
instruments. Third, NASA’s development
of the light and relatively cheap ASRG plutonium-based power systems enables cheaper
missions than were possible with the older, heavier power systems. And fourth, the changing outer planet
alignments have made gravity assists from Jupiter and Saturn to shorten flight
times to Neptune impossible the current mission planning window. Jupiter is still available for Uranus
missions in the coming decade.

Voyager 2’s cameras saw a
near featureless cloud deck at Uranus.
More recent images from observatories looking in different portions of
the spectrum have shown that Uranus has an atmospheric circulation system as
complex as Jupiter’s or Saturn’s. From a
presentation on a Uranus atmospheric probe mission by Mark Marley and
colleagues at NASA’s Ames Research Center.

A presentation by Mark Hofstadter of JPL and member of a Uranus science
working group, has laid out the scientific case for a mission to this world and
its prospects. (A European vision of a
similar scale Uranus mission can be read
here.)

For Uranus, science
objectives break into several broad classes of studies. Scientists want to send a probe into the
atmosphere for detailed composition measurements of the atmosphere to better
understand how Uranus formed and how its structure has evolved. Remote observations of Uranus to study its
weather, understand its heat balance, and probe its interior are another
priority. Researchers would like to study
the planet’s ring system many and many moons up close. And finally, they would like to re-measure Uranus’
highly unusual magnetosphere which is unlike any other planet’s except for Neptune’s.

The ideal mission
architecture would combine an atmospheric probe with an orbiter, as proposed by
the Decadal Survey. (You can find a copy
of the mission study at this website.) The atmospheric probe would gather
compositional information that is not available any other way. An orbiter would stay within the Uranus
system for prolonged observations of the planet, rings, and moons. From orbit, the spacecraft would also
traverse many locations within the magnetosphere allowing a study of its
structure not possible from a flyby spacecraft that travels a single path.

The scientific case for
exploring Uranus is solid and gaining attention. NASA’s shrinking planetary science budget,
however, can no longer support the approximately $1.8B orbiter and probe
recommended in the Decadal Survey.
Mission architects, however, have begun to look at alternative, cheaper
missions that could fit in the New Frontiers (~$1B) and Discovery (~$500M)
mission programs. These cheaper missions
would reduce costs by conducting only a portion of the Uranus science goals.

The team examining
Uranus science goals for the Decadal Survey divided the science goals into Tier
1 (highest priority), Tier 2 (high priority), and Tier 3 (highly
desirable). While the Decadal Survey
endorsed a mission that would address goals from all three tiers, a New
Frontiers mission would likely focus on only the highest priorities.

Estimated
costs of Uranus mission elements from the DecadalSurvey’s Ice Giants Decadal Study Mission Concept Study. The Tier 1 (highest priority) science orbiter
costs are near the cost of a New Frontiers mission. Many observers have commented that the Decadal
Survey mission costs were conservative (i.e., erred on the high side) so a New
Frontiers Uranus orbiter might fulfill all the Tier 1 goals.

The most straightforward
way to enable a cheaper Uranus mission would be to do either an orbiter or an atmospheric
probe mission. The Decadal Survey’s minimum
orbiter mission cost (~$1.2B) was close to the close to the expected $1B cost
cap of future New Frontiers missions.
The Decadal Survey report on the conceptual design of a Uranus
orbiter-probe mission listed two Tier 1 goals:

“Determine the atmospheric zonal winds, composition,
and structure at high spatial resolution, as well as the temporal evolution of
atmospheric dynamics.” Instruments: wide angle camera with multiple
filters and a visible/near-IR (Vis/NIR) mapping spectrometer

“Understand the basic
structure of the planet’s magnetosphere as well as the high-order structure and
temporal evolution of the planet's interior dynamo.”

An
orbiter would allow many regions of the Uranus system to be explored. This figure is from the mission concept study
and shows the path of the orbiter in a Uranus-fixed coordinate system.

If the spacecraft’s orbit were
designed to enable close flybys of Uranus’ larger moons, these three
instruments would also address two Tier Two science priority objectives: “Remote
sensing observations of large satellites,” and, “detect induced magnetic
fields that would be indicative of interior oceans [in Uranus’ moons].” However, the best orbits for meeting the
Uranus observations are not the same as those for close flybys of the
moons. Accomplishing both would require
a longer and more expensive orbital tour (~$26M more) that might not fit within
a New Frontiers budget.

The three Tier 1 science
instruments are based on mature technologies, and similar instruments have
flown on numerous missions. They could
have a combined mass less than 15 kg. By
comparison, just the Cassini Saturn orbiter’s camera system is almost 58 kg and
is just one of twelve instruments. A
Uranus mission that focused just on Tier 1 science would indeed be a tightly focused
science mission. (If money for a the
full $1.8B Flagship mission were to become available, the mission would add
fields and particle sensors, a mid-infra-red thermal detector to measure the
distribution of thermal emissions, a narrow-angle camera, and a UV imaging
spectrograph, and a nephelometer to detect clouds to the atmospheric probe.)

To my knowledge, no
formal analysis of a New Frontiers class Uranus atmospheric probe mission has
been performed. However, the
requirements and complexity of such a mission would seem to be similar to that
of a Saturn atmospheric probe mission.
That mission has been studied and been judged to fit within a New
Frontiers budget. (A Uranus mission
would have a longer flight time that would raise costs somewhat compared to a
Saturn mission.) Requirements for the atmospheric probe portion of a Uranus
orbiter also were examined by as part of the Decadal Survey and continue to be
studied at NASA’s Ames Research Center.

The Decadal Survey
analysis ranked the goals for an atmospheric probe mission lower than those for
the Tier 1 orbiter science. A single
Tier 2 goal was identified for the atmospheric probe:

“Determine the noble gas abundances
(He, Ne, Ar, Kr, and Xe) and isotopic ratios of H, C, N, and O in the planet’s
atmosphere and the atmospheric structure at the probe descent location.” Instruments: Mass spectrometer and pressure, temperature, and
acceleration/deceleration sensors.

Further
analysis by the NASA Ames team suggests that the atmospheric probe should be
able to survive to below the bottom of the methane clouds where the atmospheric
pressure would be five times that at sea level on Earth. (Measurements from below the lower water clouds
– a key goal for the Galileo atmospheric probe at Jupiter – also would be
beneficial, but these are too deep and the pressure too great at Uranus to
achieve within any mission currently conceived.) Uranus is believed to contain 30 to 50 times
more carbon (a key element of methane) than would have been the mean in the
solar nebula from which the planets formed.
A key question for planetary science has been how the enrichment of
carbon and other elements occurred.
Measurements of the ratios of key elements and their isotopes would help
determine whether Uranus-class planets are cores of planets that failed to grow
into Jupiters or super-Jupiters or represent an entirely different path of
planetary creation.

The
proposed Saturn atmospheric probe would be carried to its destination by a
simple carrier craft that would also act as a data relay that would collect and
return the atmospheric probe’s data to Earth.
As envisioned by the Decadal Survey, the carrier craft would not carry
any instruments of its own. A Uranus atmospheric
probe carrier craft also probably would not carry any instruments.

If
the scientific community continues to support the same priorities as the team
that examined Uranus mission goals for the Decadal Survey, the orbiter would
address Tier 1 goals while the atmospheric probe mission would address Tier 2
goals. That would give the nod to the
orbiter mission.

There
is one programmatic hitch to using the New Frontiers program for either a
minimalistic orbiter or an atmospheric probe mission. New Frontiers missions are selected from a
list of candidate missions pre-selected by the Decadal Survey. The current list includes missions to Venus,
the moon, a comet, the Trojan asteroids, Io, and Saturn, but not Uranus. However, NASA can change the list and has
suggested (see this presentation)
that it would be open to a request by the planetary science community to change
the list in light of the smaller budgets that have become reality since the
Decadal Survey. There will also be a
science community review of the Decadal Survey in the second half of this decade
that will review the candidate list and could revise it. Either approach would add a Uranus mission to
the list of candidates, but Uranus would be in competition with five to six
other exciting destinations.

The new interest in
Uranus missions has also led mission architects to look at even lower cost
Uranus missions than those described above.
In addition to the New Frontiers program, NASA has Discovery missions
that cost approximately half that of New Frontiers missions at <$500M. To date, none of the twelve selected missions
will have flown beyond the asteroid belt.
Flying to any outer solar system destination poses considerable
challenges within the Discovery program cost cap. The long flight times require greater spacecraft
reliability and require funding many years of flight operation costs. Missions beyond Jupiter (and some within the
Jovian system) also require plutonium power supplies, although NASA in the past
has offered to help defray the costs of using an ASRG system.

To fit within the
Discovery cost cap, a Uranus mission would be limited to a simple flyby and likely
would carry only one or two instruments.
The spacecraft also would need to be able to conduct science beyond what
Voyager did in 1986, a tough task with just one or two instruments. Recently, though, a proposed Uranus Discovery
mission has been studied that would do that with just a single instrument.

To understand the
proposed mission, let me give a bit of background. Gaseous worlds have internal turbulence that
generates acoustic wave oscillations that propagate to their surface. (The closest equivalents in a solid rocky
world are earthquakes that periodically produce movement at the surface.) Solar astronomers have exploited this
phenomenon through helioseismology to study the interior “seismology” of the
sun. The giant planets, including
Uranus, can also be studied using the same technique. (Uranus is referred to as an ice giant
because the bulk of its composition (such as water) would freeze if exposed to
space that far from the sun. However,
the pressure and heat within Uranus keeps the deep interior fluid except for a
rocky core.) A Doppler spectrographic imaging instrument could measure the
oscillations and reveal the interior structures of outer planets. To date, however, no spacecraft has carried
one to an outer planet.

Acoustic
wave seismology for giant planets including Uranus. From a presentation by Steve Matousek and colleagues at
JPL on the Uranus Explorer concept.

A JPL team has proposed
a Discovery-class mission that would carry a small, 35 cm telescope that would
house both a Doppler imager and a visible camera. The spacecraft would first flyby Jupiter to
study its interior. Approximately seven
years later, the spacecraft would repeat those measurements at Uranus.

The Uranus Explorers
Doppler imager would fulfill two goals for Uranus:

Acoustic waves have been
detected on Jupiter, but not so far for the other giant planets. It may be that their greater distance puts the
detection of these waves below the measuring threshold for Earth-based
telescopes. For Uranus, these waves may
be especially weak. Acoustic waves
are created by internal motions produced by heat flow within the planet. Key trace gases such as carbon monoxide in
the upper atmosphere demonstrates that this convection occurs on planets such
as Jupiter. (Carbon monoxide forms in
the deep, hot interiors but chemical reactions in the cooler upper atmosphere
eventually converts it to methane. For
carbon monoxide to be detectable, it must be regularly replenished by
convection.) Uranus, however, is unique
among the giant planets in having a low heat flow and a lack of these key trace
gases. If the Doppler measurements fail
to measure acoustic waves at Uranus, this could put a threshold on the amount
of internal convection within the planet.

The single, quick transit of the Uranus system would not
provide time to meet all the science goals for studying the moons and rings. However, during the hours around closest
approach, the visible camera would address two other goals:

Determine
the geology, geophysics, surface composition, and interior structure of large
satellites.

Determine
the composition and dynamical stability of the rings and small satellites.

In researching this
post, I have come to realize how diverse Uranus’ moons are. Like Saturn, Uranus has many small moons in
and near the ring system and a diverse group of medium-sized moons further out
(see this presentation).
(Uranus, of course, lacks a truly large moon equivalent to Saturn,
though.) Uranus’ Ariel, like Saturn’s
Dione, shows signs of internal activity that have partially resurfaced its
surface. Two of Uranus’ moons are large
enough that astronomers suspect that they may harbor interior oceans.

The JPL presentation
focuses on demonstrating that a scientifically compelling Discovery-class
Uranus mission is possible. With further
study, I suspect that a proposal team building on this start would find
additional, fairly low cost ways to enhance the science. Careful design of the flyby trajectory could
have the spacecraft travel behind the rings and allow the transit of the radio
signal from the spacecraft to measure the structure of the ring system.

If an ultra-stable
oscillator was including in the communications system, the radio transmissions
could be used to measure gravity fields.
Improving on Voyager’s gravity measurements of Uranus itself would be
difficult because the spacecraft would have to fly dangerously close to the top
of the atmosphere where inner ring particles could present a hazard. However, the craft could be sent past one of
the moons to use gravity to measure its interior.

International
partnerships might allow adding a simple instrument or two like this to the
craft with little cost to NASA. One high
priority instrument might be a visible/near-infrared mapping spectrometer that
could measure trace gases in Uranus’ atmosphere. Measurements of trace gases (or their
absence) would provide a second way to estimate the internal turbulence of
Uranus. Alternatively, if the flyby
spacecraft carried a magnetometer and its trajectory carried it close to one of
the larger moons, it could look for changes in the magnetosphere that could
confirm an ocean. A magnetometer also would
provide a second look at Uranus’ magnetosphere.

The JPL presentation describing
the proposal calls the mission simply the “Uranus Explorer.” Because the spacecraft studies two very
different giant planets, I think a better name might be something like,
“Echoes: Giant Planet Seismology.” JPL’s
Uranus Explorer Discovery mission concept is in the early stages of study. What I find exciting is that the results so
far suggest that a compelling mission to Jupiter and Uranus could be possible
within the tight cost constraints of the Discovery program.

Editorial Thoughts:
Before I did the research for this post, I had almost given up hope for a
return to Uranus within my lifetime.
(I’m in my mid-fifties.) Now it
appears that a mission could fit within either the New Frontiers or the
Discovery program, and I’m hopeful. However,
I also recognize that NASA’s shrinking planetary science budget means fewer
missions will be flown. Competition from
other exciting solar system destinations will be fierce. But I am hopeful.

As I wrote this post, I
found myself wondering which of the mission options I personally find most
appealing. I concluded that I believe
the combination of a flyby spacecraft with the Doppler imager-camera with an
atmospheric probe would provide the combination of science that I believe is
most compelling. I don’t know if this
could be done within the cost cap of a New Frontiers mission or not, and
recognize that the scientific community may well decide that another mission
configuration provides better science.
For me, though, the combination of examining the interior of two planets,
measuring the composition of a new class of worlds with a probe, and imaging
the other half of Uranus’ moons is compelling.
But I would be excited by any mission to Uranus.

About Me

You can contact me at futureplanets1@gmail.com with any questions or comments.
I have followed planetary exploration since I opened my newspaper in 1976 and saw the first photo from the surface of Mars. The challenges of conceiving and designing planetary missions has always fascinated me. I don't have any formal tie to NASA or planetary exploration (although I use data from NASA's Earth science missions in my professional work as an ecologist).
Corrections and additions always welcome.